Variable area flowmeters also known as rotameters have been used as a flow rate monitoring device for several hundred years. One of the limitations has been that rotameters must be visually read in order to determine a flow rate and totalizing was estimated by multiplying the observed flow rate by a time interval. The accuracy of the results was dependent on a correct calibration of the rotameter and an assumption that that the flow rate remained constant over the time interval used in the calculation. When rotameters are used in industry, flow rates can and do vary considerably. Also, when totalization is needed, rotameters without a means of transmitting a signal are frequently unsatisfactory. These difficulties have become more serious in recent years with an increased need for greater reliability and precision. This is particularly true for industries such as semiconductor and pharmaceutical manufacture.
Rotameters have been disclosed with monitoring systems designed to address the problem associated with a need for visual observation. However most of theses systems are at least partly mechanical and contain moving parts that decrease reliability because they are exposed to mechanical failures. A few systems are photoelectronic without moving parts. One, using Charged Coupled Device (CCD) technology has an inherently limited physical range of use that increases system complexity and reduces precision. Another, employing a float shadow detection means and widely spaced light sensors requires complex programming and circuit design and does not achieve precision greater than that available by visually monitoring a graduated scale on the chamber of the flowmeter.
There is still a need for a rotameter system that is more reliable and has greater precision. There is also a need for a more reliable and precise system that can also be retroactively used with existing rotameters.